1. Field of the Invention
The present invention relates to electric power steering systems.
2. Description of the Related Art
Conventionally, an electric power steering system (EPS) is known which applies power of an electric motor to a steering mechanism of a vehicle to assist driver's steering operation. For example, an EPS of Japanese Patent Application Publication No. 2014-040179 (JP 2014-040179 A) includes a control device that calculates an assist command value based on the steering torque and the vehicle speed to control driving of a motor based on the assist command value. More specifically, the control device calculates a first assist component that is a basic component of the assist command value based on the steering torque and the vehicle speed. The control device also calculates a steered angle command value based on the steering torque and the first assist component and calculates a second assist component through feedback control in which an actual steered angle is caused to match the steered angle command value. The control device calculates the assist command value by adding the second assist component to the first assist component.
The control device adds the first assist component and the steering torque to obtain drive torque, and calculates from the drive torque the steered angle command value based on an ideal model. The ideal model includes an EPS-side ideal model that depends on characteristics of each element of the EPS such as a steering shaft and the motor, and a vehicle-side ideal model that depends on characteristics of the vehicle on which the EPS is mounted. The EPS-side ideal model is formed by a viscosity term that is proportional to a first-order time differential value of the steered angle and an inertia term that is proportional to a second-order time differential value of the steered angle. The vehicle-side ideal model is formed by a spring term that is proportional to the steered angle. The drive torque is modeled as the sum of the spring term, the viscosity term, and the inertia term.
In order to achieve a more appropriate steering feel, it is necessary to balance the values of the spring term, the viscosity term, and the inertia term. However, since the values of the spring term, the viscosity term, and the inertia term are determined independently, the following problem may occur depending on the balance among the values of the terms. For example, the spring reaction force based on the spring term is so strong that the viscosity reaction force (damping) based on the viscosity term is weak accordingly. In this case, the driver is less likely to feel the viscosity as a change in the steering torque. On the contrary, the viscosity reaction force based on the viscosity term is so strong that the spring reaction force based on the spring term is weak accordingly. In this case, the driver more strongly feels the viscosity as a change in the steering torque.